This invention relates to magnetron tuning systems for magnetrons having a closed end and a proscribed internal radius.
At present there are two knonw main ways of frequency tuning such anodes.
The first method is shown in FIGS. 1A and 1B. FIG. 1A shows a cross-section through one side of a magnetron 1 along the line Y--Y in FIG. 1B, while FIG. 1B shows a cross section along the lines X--X in FIG. 1A.
The magnetron 1 is formed by a cylindrical outer body 2 bearing a plurality of vanes 3 on its inner surface. The magnetron 1 has a closed end 4. The volume between the vanes 3 defines the interaction space of the magnetron 1 and thus the resonant frequency which is dependent on it. An internal radius 5 of the magnetron 1 is proscribed from housing a frequency tuning mechanism because it is filled by an on-axis assembly, such as a cathode support, and this proscribed section extends along the axis of the magnetron beyond the closed end 4. A conductive plunger 6 having a number of conductive arms 7 occupying the volume between the vanes 3 and in electrical contact with adjacent vanes is used to tune the magnetron 1.
This tuning is carried out by moving the plunger 6 axially along the magnetron 1, for example to the dotted position 8. This alters the length of the interaction space by short circuiting the vanes 3 and so alters the resonant frequency of the magnetron.
A first bellows arrangement 28 links the plunger 6 with the magnetron 1 and a second bellows arrangement 29 links the plunger 6, with an extension (not shown) of the magnetron 1. This double-bellows arrangement prevents movement of the plunger 6 due to atmospheric pressure changes.
A cylindrical extension member 30 of the plunger 6 bearing a screw thread on its outer surface is secured to the plunger 6. A second cylindrical member 31 bearing a screw thread on its inner surface is attached to the magnetron 1 by a bearing allowing it to rotate relative to the magnetron 1 but not allowing axial movement relative to the magnetron 1. The second cylindrical member 31 can be rotated relative to the magnetron 1 by an electric motor. This bearing and driving arrangement is omitted for clarity. The threaded surfaces of the two cylindrical members 30 and 31 co-operate such that when the second cylindrical member 31 rotates, the first cylindrical member 30 is moved axially relative to the magnetron 1.
The position of the plunger 6 can thus be altered by operation of the motor.
There are a number of disadavantages to this arrangement. When the length of the interaction space is altered the dynamic impedance of the magnetron 1 will alter and, as a result, the voltage and power of the magnetron 1 will alter. Therefore, in order to keep the magnetron 1 stable a feedback system controlling the power supply must be used in conjunction with the tuner. The interior of the magnetron 1 is very hot so thermal expansion and contraction of the plunger 6 after it has been pushed further into or out of the magnetron 1 will cause the resonant frequency of the magnetron 1 to alter with time, making further tuning necessary. Further, in order to resist the thermal stresses produced by the heat of the magnetron 1 the plunger 6 must be relatively massive and so any non-axial accelerations acting on the magnetron 1, due to vibration for example, will pull the plunger 6 off axis and this will alter the size of the interaction space and de-tune the magnetron 1.
A second method of tuning magnetrons is shown in FIG. 2 which shows a cross-section through a magnetron.
A magnetron is formed by a cylindrical outer body 2 bearing a plurality of vanes 3. A conductive pin 9 is electrically linked to a vane 3A adjacent to a first cavity 14. The conductive pin 9 passes through a hole 10 in the outer body 2 of the magnetron 1 and into a second cavity 11.
The second cavity 11 is formed by a conductive tube 12 and a conductive plunger 13.
When the plunger 13 is moved along the tube 12 the length, and thus the resonant frequency, of the second cavity 11 is altered. Since the second cavity 11 is linked to the first cavity 14 this alteration of the resonant frequency of the second cavity 11 will alter the resonant frequency of the first cavity 14.
This method of tuning has the disadvantage that the azimuthal symmetry of the magnetron 1 is destroyed, resulting in a reduction in the frequency stability of the magnetron 1.
Both of these methods have the drawback that they tune all modes simultaneously. This can be a problem because magnetrons resonate in a plurality of modes, each mode generally having a different frequency. Simultaneous tuning of all modes can result in modes at unwanted frequencies entering the output frequency band of the transmitting system fed by the magnetron and producing spurious extra signals.